Selective electronic tripping element

ABSTRACT

The instantaneous tripping threshold of the selective tripping device is a decreasing function of a quantity representative of the time necessary for the current to reach the peak value. The variation curve is disposed between first and second curves, obtained experimentally, representative of envelopes of the peak current values. The first curve is obtained when the circuit breaker associated to the selective tripping device is alone and the second curve when it is connected in series with another circuit breaker connected down-line.

BACKGROUND OF THE INVENTION

The invention relates to a selective electronic tripping deviceassociated to a circuit breaker and comprising:

means for measuring the current flowing in the circuit breaker,

processing means connected to the means for measuring the current andcomprising means for determining a quantity representative of the peakvalue of the current, means for comparing said quantity with aninstantaneous tripping threshold so as to provide an instantaneoustripping signal when said quantity exceeds said threshold.

Known electronic tripping devices comprise a processing unit, generallywith a microprocessor, performing in particular long delay, short delayand instantaneous tripping functions. To achieve selectivity between twocircuit breakers connected in series or cascading, it is known toprovide a high rating for the up-line circuit breaker and a lower ratingfor the down-line circuit breaker. In this case the instantaneoustripping selectivity is of the current intensity type, the instantaneoustripping threshold of the down-line circuit breaker being lower thanthat of the up-line circuit breaker.

It is also known to use a time-based selectivity between two circuitbreakers. In this case the tripping device of the up-line circuitbreaker has a greater instantaneous tripping delay than theinstantaneous tripping time of the tripping device of the down-linecircuit breaker.

These two types of selectivity are not always efficient whenshort-circuit currents are very high, since high current thresholds arereached in both circuit breakers and a too long time delay of theassociated tripping device would be liable to lead to premature wear ofthe contacts of the up-line circuit breaker.

To reduce these drawbacks a selective tripping device has been proposed,in particular in the document EP-A-128084, comprising a counter enablingthe successive opening and closing cycles of the circuit breakercontacts in case of a short-circuit to be counted.

It has also been proposed, in particular in the document EP-A-872939, todisable an instantaneous tripping order if repulsions are detected forprimary currents lower than a preset threshold.

Another type of selective instantaneous tripping, which will bedescribed in greater detail with respect to FIGS. 3 and 4, uses one timedelay when the current has exceeded a first, low, threshold, and asecond, higher, threshold after this time delay.

In all known selective tripping devices, it is in practice necessary towait for at least two short-circuit current peaks before making aninstantaneous tripping decision. This leads to detrimental wear of thecircuit breaker contacts.

OBJECT OF THE INVENTION

The object of the invention is to provide a selective tripping devicenot presenting these shortcomings. The selective tripping deviceaccording to the invention has to be faster than known selectivetripping devices, while at the same time achieving selectivity with adown-line circuit breaker so as to reduce wear of the contacts as far aspossible in the event of a short-circuit.

This object is achieved by the fact that the processing means comprisemeans for determining a second quantity, representative of the timenecessary for the current to reach the peak value, and means fordetermining the instantaneous tripping threshold according to adecreasing function of the second quantity.

According to a development of the invention, the instantaneous trippingthreshold varies according to the second quantity according to a curvedisposed between first and second curves representative of envelopes ofthe peak values of the current, the first curve being obtained when theassociated circuit breaker is not connected in series with anothercircuit breaker connected down-line and the second curve being obtainedwhen the associated circuit breaker is connected in series with anothercircuit breaker connected down-line. The first and second curves arepreferably determined experimentally.

The tripping device according to the invention thus enables theassociated circuit breaker not to be opened for no purpose when thelatter is connected in series with a down-line circuit breaker. However,in the absence of a down-line circuit breaker, the absence of the latteris detected in less than one current half-period, which enables thedestructive effect of repulsion of the contacts in case of ashort-circuit to be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments, given asnon-restrictive examples only and represented in the accompanyingdrawings in which:

FIG. 1 represents, in block diagram form, a tripping device of knowntype wherein the invention can be implemented.

FIG. 2 schematically illustrates a particular embodiment of anelectrical installation comprising circuit breakers connected in serieswherein the tripping device according to the invention can be used.

FIGS. 3 and 4 illustrate operation of a selective instantaneous trippingdevice, of known type, using a time delay.

FIGS. 5 and 6 represent the variations, versus time, of the primarycurrent Ip (unbroken lines) for two different values of the prospectiveshort-circuit current (dotted lines), in a tripping device of known typehaving a certain limiting capacity.

FIG. 7 represents two families of curves representative of thevariations, versus time, of the primary current Ip, measuredexperimentally in a single circuit breaker (dotted lines) and,respectively, in a circuit breaker connected in series with anothercircuit breaker located down-line (unbroken lines).

FIG. 8 represents the envelopes E1 and E2 of the peak values of twofamilies of curves, respectively with a single circuit breaker and withtwo circuit breakers connected in series, versus the time tc when thefirst peak of the current Ip occurs, and a curve E3 representative ofthe instantaneous tripping threshold according to the invention.

FIGS. 9 to 12 illustrate various embodiments of the curve representativeof the value S of the instantaneous tripping threshold according to theinvention versus the time tc necessary for the current to reach the peakvalue.

FIG. 13 represents the variations, versus time, of the current Imeasured in the tripping device according to the invention.

FIG. 14 is an alternative embodiment of FIG. 13 wherein the trippingdevice performs measurement of the current I by sampling.

FIGS. 15 and 16 illustrate two alternative embodiments of a processingflowchart able to be used in a tripping device according to theinvention.

FIGS. 17 and 18 represent two particular embodiments, of known type, ofa current sensor able to be used in a tripping device according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, contacts 1 of a circuit breaker enable the current flow inthe conductors of an electrical power system 2, a three-phase system inthe particular embodiment represented, to be interrupted. An electronictripping device associated to the circuit breaker comprises a processingunit 3 connected to current sensors 4 a, 4 b and 4 c supplying signalsrepresentative of the primary currents flowing in the power systemconductors. The processing unit 3 can perform various tripping functionsand supplies a tripping signal, when required, to an opening relay 5 ofthe contacts 1 of the associated circuit breaker.

The processing unit 3 represented in FIG. 1 comprises a processingcircuit 6, preferably a microprocessor-based circuit, receiving signalsI representative of the measured currents, possibly via a shapingcircuit 7. The latter circuit performs in particular rectifying of thesignals supplied by the current sensors.

The processing unit also comprises a power supply circuit 8 whichsupplies the processing circuit with a suitable supply voltage Vdc. Inthe embodiment represented in FIG. 1, the tripping device isself-powered, i.e. the power supply circuit 8 is supplied by the currentsensors 4 a to 4 c via the shaping circuit 7. The power supply Vdcdisappears after opening of the circuit breaker.

The electrical installation represented in FIG. 2 comprises a firstcircuit breaker 9, or up-line circuit breaker, connected to a main line10 and supplying a secondary line 11, itself connected to two down-linecircuit breakers 12 and 13.

If a short-circuit occurs on a line 14 supplied by a down-line circuitbreaker, the circuit breaker 12 in FIG. 2, the up-line circuit breaker 9detects the fault. However it has to have a selective behavior and, inthis case, must not trip immediately. The down-line circuit breaker 12in fact also detects the fault, opens and annuls the short-circuitcurrent. The selective up-line circuit breaker 9 no longer detects afault, does not trip, and continues to supply other circuit breakers orapparatuses, such as the circuit breaker 13.

A short-circuit occurring on the line 11, supplied directly by theup-line circuit breaker 9, is detected only by the up-line circuitbreaker 9. In this case, the up-line circuit breaker 9 has to trip asquickly as possible.

In the prior art, selectivity of the up-line circuit breaker isachieved, for example, by a selective tripping device, associated to theup-line circuit breaker and using a time delay. The operation of such atripping device will be described briefly with reference to FIGS. 3 and4.

The processing circuit 6 of the selective tripping device compares thecurrent I with a low threshold Sb. When this threshold is reached, at atime t1 (FIG. 3), a time delay signal A changes value. For example, thesignal A goes from 0 to 1 at the time t1 in FIG. 4. The signal A keepsthis value, 1, during a preset time delay duration T then reverts to itsinitial value, 0, at a time t2. In a preferred embodiment, the timedelay has a duration T=8.125 ms, for a power system operating at 50 Hz.

Throughout the duration of the time delay, the selective tripping devicecannot trip. This does however enable a down-line circuit breaker, ifthere is one, to clear a fault during this time. At the end of the timedelay, the selective tripping device compares the current I with a highthreshold Sh>Sb. If a down-line circuit breaker has cleared the faultduring the time delay, before the time t2, the current I measured by theselective tripping device does not exceed the threshold Sh. The faultmay on the other hand not have been cleared at the time t2, eitherbecause there is no down-line circuit breaker, or due to malfunctioningof the latter, or because the fault does not affect the down-linecircuit breaker (short-circuit on the line 11 of FIG. 2 for example). Inthis case, as represented in FIG. 3, the current I exceeds the highthreshold Sh and the selective tripping device causes opening of theup-line circuit breaker. Tripping of the up-line circuit breaker hasthus been delayed and this delay, which is for example about 14 ms,causes wear of the circuit breaker. The fault current flowing in theup-line circuit breaker is never broken before it has reached its peakvalue twice. In practice, depending in particular on the samplingfrequency, it is even often necessary to wait for three passages via thepeak value before opening of the up-line circuit breaker takes place.

The object of the invention is to eliminate this drawback. The selectivetripping device according to the invention must be able not to trip ifthe down-line circuit breaker is present and to trip very quickly, inless than one half-period if the down-line circuit breaker is absent, soas to minimize the destructive effect of repulsion of the contacts ofthe up-line circuit breaker and to increase the number of breakingoperations able to be performed by this circuit breaker, wherein theterm “repulsion” describes a mechanical displacement of thecircuit-breaker contacts due to a short-circuit current flowing in thecircuit-breaker.

The invention takes certain particularities of the primary currentflowing in the up-line circuit breaker in the event of a short-circuitinto account.

It is known that in the event of a short-circuit, a circuit breaker hasa certain limiting capacity, more or less great depending on the type ofcircuit breaker. FIGS. 5 and 6 illustrate the variations, versus time,of the primary current Ip for two different values of the prospectiveshort-circuit current. In FIG. 5, a prospective short-circuit current of150 kA rms (dotted line) gives a limited current having a peak value of35 kA and a duration of 3 ms. In FIG. 6 on the other hand, a prospectiveshort-circuit current lower than the previous one, for example 100 kArms (dotted line) generates a limited current having a peak value of 30kA, slightly lower than the previous value, and a duration of 6 ms,greater than the previous duration.

FIG. 7 represents two families of curves obtained experimentally andrepresenting the variations, versus time, of the primary current Ip in acircuit breaker in the event of a short-circuit. The experimental curvesof FIG. 7 represent the actual variations of the primary current versustime, during a short-circuit (to be compared with the curves of FIGS. 5and 6). The first family of curves, in dotted lines, corresponds to asingle circuit breaker. The second family of curves, in unbroken lines,corresponds to the primary current flowing in the up-line circuitbreaker when the short-circuit current flows in a down-line circuitbreaker, with which the up-line circuit breaker is connected in series,resulting in interrupting the circuit by the down-line circuit breaker,of lower rating than the up-line circuit breaker, and limiting of theshort-circuit current flowing in the two circuit breakers. Within eachfamily of curves, the different curves are obtained for different valuesof the prospective short-circuit current and, as in FIGS. 5 and 6, theduration of the primary current increases when the amplitude of thefirst current peak decreases.

It can also be observed that, for the same prospective short-circuitcurrent, the first peak occurs more quickly when the down-line circuitbreaker has interrupted the circuit (unbroken line) than when theup-line circuit breaker is alone (dotted line). In FIG. 7 the envelopesof the peak values of the two families of curves have been representedin bold lines.

Tests were carried out modifying, for each family of curves, both theprospective short-circuit current and the angle of closing (offset withrespect to zero crossing) of the test, so as to take into account thedifferent cases able to be envisaged for a short-circuit (amplitude,symmetrical, asymmetrical short-circuit, etc . . . ). In this way, theenvelopes of the peak values Ic of two families of curves can be plottedfor each circuit breaker, versus the time tc when the first current peakoccurs. The experimental curves of FIG. 8 represent the envelopes of thepeak values of the curves of FIG. 7 versus the time period tc, wherebytc corresponds, for each curve of FIG. 7, to the time period necessaryto attain the first current peak lp (delay of its appearance), as shownon FIG. 7. In FIG. 8, the curve E1 corresponds to an envelope obtainedwith a single circuit breaker and the curve E2 to an envelope obtainedwith up-line and down-line circuit breakers connected in series. Twovalues of the peak current amplitude Ic are thus associated to a timetc1 when the first peak occurs, a first value Ic1 corresponding to thecase where the circuit breaker is alone (curve E1) and a second valueIc2 corresponding to the case where the circuit breaker is connected inseries with a down-line circuit breaker (curve E2). At each time tc1when the first peak occurs, the second value Ic2 is lower than the firstvalue Ic1.

The invention uses this property to enable the selective tripping deviceto differentiate, when a short-circuit occurs, between the case wherethe associated circuit breaker is alone and the case where it isconnected in series with a down-line circuit breaker and to adapt itsinstantaneous tripping strategy accordingly.

For this purpose an instantaneous tripping threshold S is defined whichis a decreasing function of the time tc when the current signal peakoccurs. In FIG. 8, a curve E3 represents this function S(tc). The curveE3 is disposed between the curves E1 and E2. Thus a threshold value (S1)comprised between the associated values (Ic1, Ic2) of the curves E1 andE2—Ic2<S1<Ic1—corresponds to each value (tc1) of the time when the firstpeak occurs.

The distance between the curves E1 and E2 is all the larger the greaterthe difference between the circuit breaker ratings. This distance alsoincreases with the limiting capacity of the down-line circuit breaker.The higher the limiting capacity of the down-line circuit breaker, thelower the corresponding peak values. An optimum difference is obtainedwith a down-line circuit breaker having a very good limiting capacity,and an up-line circuit breaker without any limiting capacity, the ratingof the up-line circuit breaker being at least twice the rating of thedown-line circuit breaker.

The values Ic are representative of the peak values of the primarycurrents. The selective tripping device in practice uses, in veryconventional manner, the values measured by the current sensors.

According to a first embodiment of the invention, the function S(tc) isa step-wise function, as represented in FIGS. 9 and 10. This type offunction is more particularly suitable for storing in table form in amicroprocessor-based tripping device.

In FIG. 9, two curves S(tc) are represented, the curve E3 b beingdisplaced along the time-axis with respect to the curve E3 a. The curveE3 a is used in self-powered tripping devices (FIG. 1) in which theprocessing circuit power supply is only provided via current sensors,i.e. when the processing unit is not powered beforehand. The curve E3 bis used in the case where the tripping device is always supplied (priorpower supply of the processing unit) for example by means of anauxiliary power supply (or when the circuit breaker was alreadyoperating at rated current at the time the short-circuit occurred).

In FIG. 10, a curve E3 d is shifted in amplitude in a positive thresholdS-direction with respect to a curve E3 c.

For example purposes, the curve E3 c is used in a tripping deviceassociated with a circuit breaker of 250 A rating, whereas the curve E3d is associated with a circuit breaker of higher rating, for example 630A. For the same value of the time tc, the threshold corresponding to thecurve E3 d is greater than the threshold corresponding to the curve E3c.

It is possible to provide a plurality of tables in the selectivetripping device corresponding to the various possible options (with orwithout prior power supply, different ratings) and to validate theoption chosen in the tripping device when performing installationthereof.

According to another embodiment of the invention (FIG. 11), the functionS(tc) can be constituted by a plurality of segments of straight lineswith different slopes. As in FIGS. 9 and 10, this function can bestaggered in time or in amplitude according to the various possibleoptions.

The same is the case for the embodiment of FIG. 12 in which the functionS(tc) is obtained from a mathematical function. As FIGS. 9-12 show, theinstantaneous threshold S(Tc) decreases when Tc is increasing, i.e.,that a lower value of S(Tc) corresponds to a greater value of Tc.

The operation of a particular embodiment of a tripping device accordingto the invention will be explained hereafter, with reference to FIG. 13which represents the variations, versus time, of the current I suppliedto the processing circuit 6 of the tripping device (in the absence oftripping).

In this particular embodiment, the processing circuit compares thecurrent I with a low threshold Sb. As soon as this threshold is reached(time t3), the tripping device initializes counting of the time Tcnecessary for the current I to reach its first peak value Ic (time t4).By using a predetermined curve S(tc) of the type defined above, thetripping device accordingly adapts the instantaneous tripping thresholdto a value S(Tc). If, at the time t4, the current Ic exceeds thethreshold S(Tc), the associated circuit breaker is considered to bealone and a tripping order is immediately generated. If on the otherhand, at the time t4, the current Ic remains lower than the thresholdS(Tc), then the tripping device considers that the associated circuitbreaker is connected in series with a down-line circuit breaker and doesnot trip, the down-line circuit breaker then clearing the fault.

Thus, in the absence of a down-line circuit breaker, there is no timedelay for tripping of the circuit breaker connected on its own and wearof this circuit breaker is thereby reduced. The no time delay decisionbeing taken as soon as the first peak of the signal (t4) occurs, thenumber of breaks on short-circuit able to be performed by a circuitbreaker can be increased from 2 or 3 in the prior art to 6 to 9 with theselective tripping device according to the invention.

According to a preferred embodiment, the processing circuit of thetripping device performs current measurement by sampling as representedin FIG. 14. The current is sampled with a preset sampling period Te.

According to the flowchart of FIG. 15, processing begins with aninitialization step F1 during which the quantity Tc, representative ofthe time necessary for the current to reach the peak value, is reset tozero. Then the processing circuit performs (F2) measuring of a currentsample. It then compares (F3) the amplitude of this sample with the lowthreshold Sb. If this amplitude is lower than or equal to Sb (no outputof F3), the circuit goes back to step F2. If the threshold Sb has beenreached (yes output of F3), the processing circuit increments (F4) thevalue of Tc:Tc=Tc+1. Thus when the amplitude of a first sample (I1 inFIG. 14) exceeds the threshold Sb, the quantity Tc takes the value 1.Then the processing circuit determines (F5) the value S(Tc) of thethreshold associated to this value of Tc. It then examines (F6) whetherthe peak has been reached. If this is not the case (no output of F6), itperforms (F7) a new measuring of I before returning to the step F4. Ateach new sample, the quantity Tc is incremented and the correspondingthreshold determined. When the peak is reached (yes output of F6), theprocessing circuit compares the last sample with the threshold S(Tc). Ifthe threshold is not exceeds (no output of F8), the processing iscompleted, the short-circuit being dealt with by the down-line circuitbreaker. If on the other hand the threshold is exceeded (yes output ofF8), the processing circuit triggers (F9) tripping of the circuitbreaker.

In the alternative embodiment represented in FIG. 16, the step F5 iseliminated and replaced by an equivalent step, F10, performed betweenthe steps F6 and F8. Thus, in the first embodiment, the threshold iscontinuously adapted to the last value of the quantity Tc measured. Inthe second embodiment on the other hand, the threshold is onlydetermined when the peak has been reached, which minimizes theprocessing time at each sample.

In a preferred embodiment, the peak is considered as being reached whenthe current value drops back, i.e. when the amplitude of the last sampleis smaller than that of the previous sample. In FIG. 14, the amplitudeof the 5^(th) sample (I₅) is smaller than that of the previous sample(I₅<I₄). The peak is therefore considered as being reached when thequantity Tc is equal to 5, which is representative of 5 times thesampling period Te. The threshold corresponding to this time is thenused to make a decision to perform instantaneous tripping, if required.

For the results obtained to be reliable the current supplied by a sensorhas to be representative of the primary current. The lower the remanenceof the sensor the less it saturates, and the more the results arereliable. FIGS. 17 and 18 represent two types of current sensor of knowntype fulfilling these conditions satisfactorily.

The current sensor according to FIG. 17 is a linear sensor. It comprisesa magnetic circuit and a secondary winding formed by a coil 16. Themagnetic circuit, generally formed by stacked plates, surrounds theconductor 17 of the power system where the primary current to bemeasured is flowing. A part 18 of the magnetic circuit 15 passes throughthe center of the secondary winding and forms the core of the coil 16.

The preferred embodiment of the current sensor represented in FIG. 18 isan iron sensor with shunt effect of the type described in the documentEP-A-704867. It comprises a magnetic shunt 19, branch connected on themagnetic core of the secondary winding and comprising an air-gap 20. Acurrent sensor of this kind presents in particular a lower remanencethan that of other types of sensor, which makes it more reliable formeasuring the first peak.

The invention is however not limited to a particular type of currentsensor. Other types of linear current sensor can notably be used. Forexample Air current sensors, Iron current sensors, Hall effect currentsensors, etc. can be used.

The invention applies equally well to a single-phase or a multi-phasepower system. In the latter case, processing is performed separately foreach phase.

The low threshold Sb (FIGS. 13 and 14) is preferably different fromzero. The curves S(tc) are adapted to take account of the value of Sbchosen.

What is claimed is:
 1. A selective electronic tripping device for acircuit breaker, said device comprising: current measuring means formeasuring current flowing in a circuit breaker; processing meansconnected to the current measuring means and comprising firstdetermining means for determining a first quantity representative of apeak value of such current; means for comparing said first quantity withan instantaneous tripping threshold for providing an instantaneoustripping signal when said first quantity exceeds said threshold; whereinsaid processing means further comprises second determining means fordetermining a second quantity representative of an amount of timenecessary for such current to reach the peak value, and thirddetermining means for determining the instantaneous tripping thresholdaccording to a decreasing function of the second quantity.
 2. Thetripping device according to claim 1, wherein said processing means isfor calculating the instantaneous tripping threshold according to thesecond quantity and predetermined values defining a threshold variationcurve disposed between first and second curves representative ofenvelopes of the peak values of a current flowing in a circuit breaker,the first curve having been predetermined when a first circuit breakerassociated with the tripping device was not connected in series with asecond circuit breaker connected down-line, and the second curve havingbeen predetermined when a first circuit breaker was connected in serieswith a second circuit breaker connected down-line.
 3. The trippingdevice according to claim 1, wherein the second quantity isrepresentative of time necessary for a current flowing in a circuitbreaker to reach the peak value from a moment such current exceeds apreset low threshold.
 4. The tripping device according to claim 1,wherein said function is a step-wise function.
 5. The tripping deviceaccording to claim 1, wherein said function comprises a linearpiece-wise approximation.
 6. The tripping device according to claim 1,wherein, for a preset second quantity, the amplitude of theinstantaneous tripping threshold increases with a rating of anassociated circuit breaker.
 7. The tripping device according to claim 1,wherein said processing means further comprises fourth determining meansfor determining said instantaneous tripping threshold from one of twosets of predetermined values, a first set thereof defining a firstcurve, applicable when a short-circuit occurs when the processing meansare not powered, and from a second set of values defining a second curveapplicable when a short-circuit occurs when the processing means arepowered, the second curve corresponding to the first curve but displacedalong the time-axis relative to the first curve.
 8. The tripping deviceaccording to claim 1, wherein the means for measuring a current flowingin a circuit breaker comprise at least one linear current sensor.
 9. Thetripping device according to claim 1, wherein the means for measuring acurrent flowing in a circuit breaker comprise at least one iron sensorwith shunt effect.